1. Field of the Invention
The present invention relates generally to dynamically controlling spectral power distributions within optical networks and particularly to controlling spectral power distributions between channels of wavelength division multiplexing systems.
2. Technical Background
Wavelength division multiplexing (WDM) systems, which convey a number of different wavelength channels, undergo various changes that unequally affect the transmission of the different channels. Wavelength-dependent propagation losses and repeater gains, system aging, environmental influences, and the addition or substitution of new wavelength-dependent components can all affect channel power distributions.
Such wavelength-dependent variations accumulating along WDM optical systems, sometimes referred to as "ripple", lead to dissimilar received signal power among the channels and a worsening signal-to-noise ratio (SNR).
Since the weakest signal (channel) must be received with a reasonable SNR, system ripple constrains the maximum system reach.
Permanent spectral power adjustments are often incorporated into optical transmission systems. For example, some optical amplifiers are packaged with gain flattening filters to compensate for uneven gain profiles of the amplifiers. Maintenance adjustments, referred to as "trimming", are sometimes made in established systems to compensate for planned or incidental changes to system profiles. We have recognized that optimized system performance can require continuous or periodic adjustments that compensate for less predictable or temporary fluctuations in the spectral response.
Tunable filters, particularly tunable fiber Bragg gratings, are available with spectral responses that can be shifted along the spectrum. Filter gratings are tuned by varying their periodicity under the control of an external force such as compression or stress. However, the system spectral transmission characteristics that vary over time are not easily counteracted by the shifting of narrow attenuation bands. Especially with respect to closely spaced signals along the spectrum, shifting attenuation bands can disturb adjacent signals.